In portable equipment, such as mobile telephones, prismatic shaped batteries are generally preferred to rod shaped ones in particular to reduce the overall size of the equipment. This portable equipment, which must by definition be light, generally incorporate electronic circuits and components which are fragile and often demanding from the point of view of power consumption. It is thus desirable for the prismatic batteries used in such equipment to have high power density while being light and of the lowest manufacturing cost possible, for them to have sufficient longevity, in particular in the case of storage batteries, and for them to have during use all necessary guarantees from the safety point of view, as regards both the user and the contiguous electronic circuits.
An object of the present invention is to provide a prismatic battery which, for a determined current density, is lighter, has a lower manufacturing cost than the prismatic batteries of the prior art and whose design also allows safety of use to be increased.
The prismatic batteries of the prior art, schematically shown in FIG. 1 are formed by a metal container 1 of parallelepiped shape, within which is placed a non circular spiral winding 2, of the jelly roll type (visible in FIGS. 1A, 1B), which is impregnated by an electrolyte after the winding is put in place. This winding is obtained from at least one composite strip including a layer of active material forming anode 2a, a porous separator 2b, a layer of active material forming cathode 2c and a second porous separator 2b. The anode and the cathode each include connection means 5, 6, generally arranged close to the inner wall of container 1 and at the center of winding 2. This jelly roll is generally held compressed by means of two corrugated elastic plates 3a, 3b, arranged between the large walls 11, 13 of container 1 and winding 2. These elastic plates also allow the small variations in volume of winding 2 to be absorbed during the charging/discharging cycles. The unit thereby formed is hermetically sealed by a cover 4 carrying contact terminals 7, 8 electrically connected to electrodes 2a, 2c by connection means 5, 6 said electric connections being effected by welding. This type of prismatic battery, in particular in the case of an storage battery, generally also includes a safety vent schematically shown at 9. This safety vent 9 is in fact necessary given the chemical reactions which take place and which can cause an increase in pressure and/or temperature. Because of the mechanical stress which are exerted on the case of the battery, the container was first chosen to be formed of steel sheet of equal thickness of the order f 0.5 mm (FIG. A). In order to make this battery lighter and less expensive, it was then proposed to replace the steel with aluminum or an aluminum alloy. In order to have the same resistance to deformation, it is then necessary to increase the thickness of the container wall, which has the drawback, either of increasing the size of the battery for a given power density, or reducing the quantity of active material and thus the power density if one wishes to keep the size of the battery within standard dimensions.
In order to avoid this drawback, U.S. Pat. No. 5,556,722 proposes reinforcing the angles 1a to 1d of the container, i.e. having a excess thickness locally, as shown in FIG. 1 B. Although the manufacturing method for such a container is not described, it is obvious that such a configuration increases the cost of the final product given that one can no longer use a metal sheet of equal thickness as the starting material.